This invention relates to devices which detect and/or treat tachyarrhythmias (rapid heart rhythms), and more specifically, to mechanisms to distinguish among various tachyarrhythmias and to provide appropriate therapies to treat the identified tachyarrhythmias.
Early automatic tachyarrhythmia detection systems for automatic cardioverter/defibrillators relied upon the presence or absence of electrical and mechanical heart activity (such as intra-myocardial pressure, blood pressure, impedance, stroke volume or heart movement) and/or the rate of the electrocardiogram to detect hemodynamically compromising ventricular tachycardia or fibrillation.
In pacemaker/cardioverter/defibrillators presently in clinical evaluation, fibrillation is distinguished from ventricular tachycardia using rate based criteria, In such devices, it is common to specify the rate or interval ranges that characterize a tachyarrhythmia as opposed to fibrillation. However, some patients may suffer from ventricular tachycardia and ventricular fibrillation which have similar or overlapping rates, making it difficult to distinguish low rate fibrillation from high rate tachycardia. In addition, ventricular fibrillation may display R--R intervals which may vary considerably, resulting in intervals that may fall within both the tachycardia and fibrillation rate or interval ranges, or outside both.
Presently available pacemaker/cardioverter/defibrillator arrhythmia control devices, such as the Model 7216A and 7217B pacemaker/cardioverter/defibrillator devices available from Medtronic, Inc., employ programmable fibrillation interval ranges and tachycardia detection interval ranges which are adjacent to one another but do not overlap. In these Medtronic devices in particular, the interval range designated as indicative of fibrillation consists of intervals less than a programmable interval (FDI) and the interval range designated as indicative of ventricular tachycardia consists of intervals less than a programmable interval (TDI) and greater than or equal to FDI. R--R intervals are counted to provide a count of R--R intervals falling within the tachycardia interval range (VTEC) and a count of intervals within the fibrillation range (VFEC). VFEC is a count of how many of the preceding series of a predetermined number (FEB) of R--R intervals is less than or equal to FDI. The VTEC count is incremented in response to R--R intervals that are greater than or equal to FDI but shorter than TDI, is reset to zero in response to intervals longer than or equal to TDI and is insensitive to intervals less than FDI. VTEC is compared to a programmed value (VTNID) and VFEC is compared to a corresponding programmable value (VFNID). When one of the counts equals its corresponding programmable value, the device diagnoses the presence of the corresponding arrhythmia, i.e., fibrillation or tachycardia and delivers an appropriate therapy, e.g., anti-tachycardia pacing, a cardioversion pulse or a defibrillation pulse. In addition, the physician may optionally program the device to require that measured R--R intervals meet a rapid onset criterion before the VTEC count can be incremented and may also optionally program the device to require that a rate stability criterion be met with each successive measured R--R interval in order to increment VTEC and that otherwise VTEC will be reset to zero. This detection system has proven effective in distinguishing between fibrillation and ventricular tachycardia so that appropriate therapies may be delivered. However, in rare instances, the detection methodology may require a sequence of a greater number of rapid heart beats than might optimally be desired to determine whether the rapid rhythm is due to fibrillation or tachycardia. Moreover, an improved level of accuracy in classifying rhythms close to FDI in average R--R interval duration is also believed desirable.